This question comes from Luke 17:21. In the passage, Jesus comment resulted from the Pharisees asking Jesus the question when the kingdom of God would come. Here is the context,
20 Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, “The kingdom of God does not come with observation; 21 “nor will they say, `See here!’ or `See there!’ For indeed, the kingdom of God is within you.” 22 Then He said to the disciples, “The days will come when you will desire to see one of the days of the Son of Man, and you will not see it. 23 “And they will say to you, `Look here!’ or `Look there!’ Do not go after them or follow them. 24 “For as the lightning that flashes out of one part under heaven shines to the other part under heaven, so also the Son of Man will be in His day. 25 “But first He must suffer many things and be rejected by this generation. (Luke 17:20-25 NKJ)
The Pharisees were expecting a physical kingdom on earth, in which the God’s reign would again make Israel as a sovereign nation. Rome controlled Israel ever since B.C. 63 and the Jewish people experienced Roman oppression financially through taxation and politically through Roman puppet kings like Herod and Pilate. The good ol’ days were long ago under Solomon, when nations sent their ambassadors and officials to see the Lord’s blessing on Israel (2 Ch. 9:1). The Israelites knew the Messiah would come and deliver them, but they did not understand the sequence of events, nor that His deliverance would first be a spiritual deliverance from sins and then the physical deliverance.
Jesus answered their question that “the kingdom of God does not come with observation.” (Luke 17:20). In other words, the coming kingdom would not be measured or observed in the physical realm. Paul mentioned this, “…for the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit.” (Rom. 14:17 NKJ) The kingdom of God is not in the physical realm of the five senses like eating and drinking are. The kingdom of God is in the spiritual realm of “righteousness and peace and joy in the Holy Spirit.” It is something that is established as a person is not merely indwelt by the Holy Spirit, but filled with the Spirit (cf. Eph. 5:18; Gal. 5:22-23). When the Holy Spirit fills, or better stated, controls, a Christian’s life, the result will be the experience of God’s presence of righteousness, peace and joy.
Hence, the kingdom of God is not a physical kingdom, but a spiritual reality in the life of the believer in Jesus Christ. However, do not wander into progressive thinking that this is Jesus ruling on the throne of David. Jesus is ruling in the hearts of His people, but the Davidic Covenant will not be fulfilled until the Second Advent, when Jesus will bodily return and rule for 1000 years. God still has a purpose for the nation of Israel and the Age of the Jews will be completed after the Church Age with the Tribulation and those believers during that Tribulational time will be ushered into the Millennium.
That is the meaning of Jesus’ comments after He discloses that the kingdom of God is a spiritual reality or relationship with eternal God. There will be people who say, “There is the kingdom of God.” Jesus warned not to follow after them, because when Jesus returns, it will be as obvious as lightning that flashes across the whole sky and everyone sees it. Jesus said they will long for the day when they will see the Messiah. They may have to wait to see it, but it will be obvious.
YET, first He must suffer and be rejected (Luke 17:25). Both aspects would be true. The nation of Israel would reject His teaching and offer of the kingdom and salvation (cf. Matt. 5-7; 11-28-30). Their rejection resulted in His suffering during His ministry and culminating in His crucifixion. The kingdom was still there as God ruled in the hearts and minds of His people.
Is Jesus ruling in your heart? Do you experience God’s righteousness, peace and joy? When Jesus is your Lord, you will be living in His righteousness and bear the Holy Spirit’s fruit of peace and joy.